I know that lions can be scared away when they see humans approaching without fear, but what if they're ambushed? How about a tiger, or a polar bear? If I manage to sneak up on one of these apex predators, and jump out, screaming, and running at it, will it run? Or will it just turn around and kill me? I guess I'm wondering if apex predators have a fight or flight reflex, and how strong it is if they do. Do they tend to resort to fighting more often if they're the baddest tiger in the jungle vs the runt of the litter? Do they quickly assess the threat and respond accordingly, or does a flight instinct kick in and send them running?
Not these guys. But what I'm not clear on is what would happen if they jumped out of the bushes behind them and yelled "BOO!". Also, lions are pretty tame and predictable compared to tigers. Tigers have a much lower bullshit tolerance.
It depends on the animal and its mood, and how territorial it is. A lion will probably freak out and run. A polar bear, or really most bears, might jump in surprise, but then either try to fuck you up, ignore you, or just wander off. Same with a moose.
I've seen black bears run from me before. They're the most docile of the bears though, and they stopped running when they were at what I assume they considered a safe distance.
I'd go for an "it depends" answer. If they are just out and about looking for food, most would probably run. No animal wants to risk injury and fighting with an unknown enemy carries too much risk. So, their instincts will likely make them run. However, if you do it near their den and/or young, the instinctual calculus may be very different and you could end up as an impromptu baby snack.